In many seat belt restraint systems employed in automotive vehicles, the retractors for the seat belts have acceleration sensors for locking the retractor upon sensing high vehicle accelerations or decelerations such as caused by vehicle impacts during accidents. With respect to vehicle acceleration sensors, there is typically an inertia member that is responsive to excessive vehicle accelerations to cause a locking mechanism to block the retractor reel against rotation preventing seat belt payout therefrom. Normally, these sensors are incorporated into the retractor. When the retractor is mounted to a vehicle seat, and particularly to an adjustable backrest thereof, the inertia sensing portion of the acceleration sensor may have to be adjusted in its position to avoid a premature actuation locking the retractor when the seat back is adjusted due to the sensitivity of the inertia member to changing attitudes.
Rather than adjusting the inertia sensor on a tiltable seat backrest, remote acceleration sensors have been mounted in the vehicle at a remote location from the adjustable backrest to which the retractor may be attached. The acceleration sensing mechanism for these remote sensing units can be of the standard mechanical type where inertia members topple or roll in response to high accelerations or decelerations causing a lock bar or pawl to engage teeth on a gear or ratchet wheel to stop reel rotation.
Where the retractor is electrically controlled for locking, such as by a solenoid device, it is known to use electrically conductive inertia members which move to close switch contacts during periods of high acceleration or deceleration. Examples of such electrically controlled remote sensors are shown in U.S. Pat. Nos. 3,915,401 and 4,708,366. While electrical inertia sensors or switches that employ switch contacts for electrically controlling operation of a retractor solenoid typically use less moving parts over mechanical sensors, they can experience problems with respect to their reliability and service life when the switch contacts are subjected to adverse environmental conditions, such as dust, temperature extremes and excessive humidity. Dust or corrosion may cause moving parts in a switch to bind to prevent opening or closing thereof, or may interrupt current flow between the contacts. In addition, over time switch contacts can wear or corrode which can adversely affect their performance. This is undesirable because, to meet automotive manufacturers' specifications, the switch must be cycled thousands of times under these adverse conditions without failure. Accordingly, the use of electrical switch contacts in inertia sensors can create problems in terms of the reliability of operation and the service life of the sensor, both of which take on critical importance due to their safety implications when employed in safety belt restraint systems.
Other important considerations that arise when selecting an inertia sensor for use in automobiles are the assembly and production costs of the sensor. In addition, the remote sensor should have a compact design so as to take up a minimum amount of space. As previously mentioned, this is particularly important with remote sensors so that they occupy as little space as possible wherever they are mounted in the vehicle interior. Accordingly, a retractor having an inertia sensor, and specifically a remote inertia sensor that avoids the use of moving switch contacts and which meets the severe operational requirements imposed by the automobile industry is desirable. The acceleration sensor must meet rigid life testing specifications and hence it should employ very reliable components. In addition, a low cost, readily assembled, compact inertia sensor that also satisfies the long life and reliability criteria would be desirable.